Step and repeat camera with computer controlled film table



Nov. 10, 1970 B. D. ABLES 3,539,256

STEP AND REPEAT CAMERA WITH COMPUTER CONTROLLED FILM TABLE Filed Oct. 18, 1967 16 Sheets-Sheet l ION I FIG.'I O (Of/44 M044 ATTORNEY STEP AND REPEAT CAMERA WITH COMPUTER CONTROLLED FILM TABLE Filed Oct. 18, 1967 B. D. ABLES Nov. 10, 1970 l6 Sheets-Sheet 5 FIG.3

HOME EXFHXTAL =O,YTAL

FIG. 3! X52 3rd EX XTAL= YTAL= B. D. ABLES Nov. 10; 1970 STEP AND REPEAT CAMERA WITH COMPUTER CONTROLLED FILM TABLE Filed Oct. 18, 1967 16 Sheets-Sheet 4.

Na OON W m @Fi vww 9m 11 K $51 a F j 0 mt v2 um mom a K o Q wk 0 v2 wt wow v2 Q .0 Q wt w wow H3 3 n k n n Y! a Q MM owww B. D. ABLES Nov. 10, 1970 STEP AND REPEAT CAMERA WITH COMPUTER CONTROLLED FILM TABLE l6 Sheets-Sheet 5 Filed Oct. 18, 1967 FIG. IO

m m F Nov. 10, 1970 B. D. ABLES 3,539,256

STEP AND REPEAT CAMERA WITH COMPUTER CONTROLLED FILM TABLE Filed Oct. 18, 1967 1,6 Sheets-Sheet 6 ONM Wm m

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l a L Even mw VEH B.. b. ABLES 3,539,256

Nov, 10, 1970 STEP AND REPEAT CAMERA WITH COMPUTER CONTROLLED FILM TABLE Filed OGL. 18, 1967 16 Sheets-Sheet '7 FIG. 23

Nov. 10, 1970 B. D. ABLES 3,539,256

v STEP AND REPEAT CAMERA WITH COMPUTER CONTROLLED FILM TABLE Filed 001:. 18, 1967 1,6 Sheets-Sheet 8 I ll I LASER '1 BEAM +4 m 0 FIG. 30

I2 MICROINCHES TABLE TRAVEL Nov. 10, 1970 L s 3,539,256

STEP AND REPEAT CAMERA WITH COMPUTER CONTROLLED FILM TABLE Filed Oct. 18, 1967 16 Sheets-Sheet 1O 700 DIAGNOSTICS COMP SELF CHECK) LOA0"NO OPERATION" 702 IN ALL INTERRUPTS EXCEPT POWER FAILURE sET UP TYPEWRITER INTERRUPTS I202 SET you)": INITIALIzE FLAGS awoRos 706 I204 "YOLD"= 250.

TYPE OUT mm 708 u I, n READ TAPES PROGRAM Is LOADED TYPE SPECIAL FORMAT,

COMPUTE LOCATION 7I2 cOLuMN a ROW HDGS. a STORE 555 k ggg n 710 TURN ON TAPE READER 704 sa "EXPOO" To zERo Y I2IO READ B BIT CHARACTER INTO "xNExT Is THIS END OF FILE READ 8 BIT CHARACTER INTO"YNExT QC) I2I4 SET BRAKE To mm. a STORE-\ IN "XNEW" TYPE "SPECIFY l I 12/6 I FORMAT" coNvERT"YNExT" To mm. a STORE IN"YNEw" CALCULATE FRINGE COUNT. POSITION 8| EXPOSE (H6235) TYPE "ERROR SET "BLANK" RETURN TO ZERO TRANSFER FORMAT I224 ATA TO'M'.N".

TYPE"REAOY To RUN" 816 TYPE "RUN COMPLETE" PUSHED? FIG. 320

TO FIG.34b

Nov. 10, 1970 B. D. ABLES STEP AND REPEAT CAMERA WITH COMPUTER CONTROLLED FILM TABLE Filed Oct. 18, 1967 FROM FIG.320

16 Sheets-Sheet l1 TYPE FORMAT NUMBER zo 113 2 H42 v I TYIPE FQRMAT INCREASE DECREASE xTAIJ' BYI s|zE MBCD x NBCD BY a9s &

TYPE SPACWQ' I I34 XSPCD'HMILSHZY SPACING YSPC MILS CALCULATE FRINGE H38 CALCULATE FRINGE COUNT POSITION a COUNT PosI'rIoN a I I EXPOE (FIG.35) EXPOE (mess) I YPE"ExRE I N v 1 EXPOSURES REQLHRED s "XTA "XUMT"? 902 I l H30 N INCREASE FlR S l E E LEJRE Y STRIKE'ELANK I ETURN TO ZERO 8 EXPOSE TYPE "REA0Y FOR EXCEPTIONS Is 'RuN BUTTON 0N n SET "YTAL"&"XTAL" TOlstEXCEPTlON CALCULATE FRINGE COUNT, POSITION a EXPOSE (FIG. 35)

CALCULATE COORDINATES CALCULATE FRINGE COUNT POSITION a EXPOE (FIG,35)

H64 SET XTAC'TO 2nd.EXCEPTlON CALCULATE COORDINATES CALCULATE FRINGE COUNT POSITION a ExPoE (FIG.34J H7O SET "YTAU'To 3rd. EXCEPTIO CALCULATECOORDlNATES FIGQ 32b H74 CALCULATE FRINGE COUNT, POSITION & EXPOSE (FIG. 35)

SET xTAIJTo 4m EXCEPTION CALCULATE FRINGE COUNT, POSITION BI EXPOSE (FIG.35)

SET "BLANK" TYPE I "RuN COMPLETE" Nov. .10, 1970 B, D, ABLE, 3,539,256

STEP AND REPEAT CAMERA WITH COMPUTER CONTROLLED FILM TABLE Filed Oct. 18, 1967 '16 Shets-Sheet 12 HOME 726 Y m 756 758 N MEASURE XVEL I056 MEASURE YVEL I096 760 Y IS XVEL IO%? 746 N 762 N 764 mmmm- SET YMOT=O MEASURE XVEL IO'lu MEASURE YVEL IO% WAIT SET Y MOT =0 x COMPARATOR INTERRUPT 'IOIO POSITIVE TRAVEL HAVE ALL EXTRA INTERRUPTS BEEN IGNORED 2 CALCULATE x RETARD a NEW x INTERRUPTIGNORE I 4 ,1020 LOAD x COMPARATOR WITH Mm XRETARD KEEP RECORD OF THIS ONE IOI6 SET UP X COMPARATOR RELOAD x COMPARATOR INTERRUPT TO so TO I x com I NEGATIVE RETARD BD'SX FIG. 39

Nov. 10, 1970 B. D, ABLE-s 3,539,256

STEP AND REPEAT CAMERA WITH COMPUTER CONTROLLED FILM TABLE Filed Oct. 18, 1967 16 Sheets-Sheet 13 TABLE RELEASE BRAKE 832 WAIT lsec.

Nov. 10, 1970 B. D. ABLES 3,539,256}

STEP AND REPEAT CAMERA WITH COMPUTER CONTROLLED FILM TABLE Filed Oct. 18, 1967 CALCULATE L6 Sheets-Sheet l4 FRINGE COUNT, POSITION BIEXPOSE 9/6 F CHECK FLAGS cALcULATE XAIM a YAIM 920 [038x N IS XNEW XOLDP Y 932 MOVE IN EQUAL MOVE IN POSITIVE x NEGATIvE x (FIG.36)

RETURN X COMP. INT POSITIVE TRAVEL (H639) SET UP SET UP MOVE IN EQUAL MOVE IN NEGATIVE Y PosITIvE Y I I RETURN L -I r J I RETURN Y COMP. INT. NEGATIVE TRAVEL I I I Y coMR INT. 92 I PosITIvE TRAVEL I046 Y COMP. INT. NEGATIVE RETARD (SLOW X2 SLOW X 2 INTERRUPT 936 FIG. 37

SET UP X COMPARATOR TO INTERRUFT ON XAIM GET PRESENT X COORDINATE [SET XMOT 20% Nov. 10, 1970 B. D. ABLES 3,539,256

STEP AND REPEAT CAMERA WITH COMPUTER CONTROLLED FILM TABLE Filed Oct. 18 1967 16 Sheets-Sheet 15 MOVE IN 932 POSITIVE X LOAD COMPARATOR WITH XAIM BRAKING DISTANCE REQUIRED BY DRIVE SYSTEM CALCULATE NUMBER OF X COMPARATOR INTERRUPTS TO IGNORE (XIGNR) SET UP COMPARATOR INTERRUPTS TO GO LTURN ROSXPILOT LIGHTON SLOW X(FIG.37)

SLOW X IIFIGBB) IS XVEL 99%? N MEASURE VELOSITY 974 ,1 I L SET XMOT I0OA j 1 988 I HAVE WE WAITED TOO LONG FOR Y THE X COMPARATOR INTERRUPTS? WAS IT FAST LAST PULSE? MAKE FORCE MORE MAKE FORCE MORE SET FORCE NEGATIVE POSITIVE I002 Ioo4 ssa FIG. 3 6 1024 HAVE ALL NEW INTERRUPTS BEEN IGNORED? I028 BEEP A RECORD 8. RELOAD comm SET XMOT 0 FIG. 40

Nov. 10, 1970 B. D. ABLES 3,539, 56

STEP AND REPEAT CAMERA WITH'COMPUTER CONTROLLED FILM TABLE L l" Sheets-Sheet l6 Flled Oct 18, 1967 POSITIONING o I DURING EXPOSURE (-)TOLERANCES. SET UP I066 FOR POSITION AVERAGING IF DEEIRED Is INDEX PUSHED? Y SET XMOT a YMOT=O l ExPosuR FINISHED READ x a Y CIPIJINTQQS a.

ROUTINE sToRE IN x a Y" IS SINGLE FLAG sET? (AND UPDATE QIERAGES) N I076 Y N I082 Y 1080 Is XAIM XAVG? I078 I086 Is YAIM YAVG? [I084 C L L TE X C= CALCULATE xFORc= CALCULATE YF0Rc= CALCULATE YFORC: |RESFO(ERROR)N+ RESFO(ERROR)N+ |REsFo(ERRoRI REsF0(ERRoR) xFRIc| XFRIC XFRICI -xFRIc L i I I sET XMOT=XFORC SET YMOT=YFORC I087 INCREMENT WITHIN 'NCREMENT TOLERANC TA ERRoR TALLY E 1,1094 IIOO N INITIALIZE ERROR TALLIES IS BLANK FLAG SET? N United States Patent 3,539,256 STEP AND REPEAT CAMERA WITH COMPUTER CONTROLLED FILM TABLE Billy D. Ables, Richardson, Tex., assignor to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware Filed Oct. 18, 1967, Ser. No. 676,100 Int. Cl. G03b 27/06 US. Cl. 35553 9 Claims ABSTRACT OF THE DISCLOSURE A film support table is supported by air bearings and is movable in the X and Y coordinate directions by X and Y drive systems utilizing precision X and Y guide rails and an air bearing keeper system. A laser interferometer and fringe counter detects movement of the table in the X and Y coordinate directions by fringe counts. A projection system simultaneously projects a plurality of images onto the film carried by the table after the table is moved to each of a plurality of predetermined exposure positions. A reference detector sys tem detects when the table is at a zero reference position and resets the counters. A digital computer is programmed to compute the coordinate of each exposure position and then, based on the current barometric pressure, compute the X and Y coordinates, in fringe counts from the reference positioning of the first exposure position. The computer then operates the drive ssytem in such a manner as to move the table to the exposure position by continuously computing the position and velocity of the table from the readings of the fringe counters. Then the table is maintained at the exposure position during the exposure period by continuously determining the position of the table from the fringe counters and operating the drive system to produce forces for correcting the positional error.

This invention relates generally to step and repeat cameras, and more particularly, but not by way of limitation, relates to such a camera for producing a set of photolithographic masks for fabricating large arrays of semiconductor devices.

A semiconductor device, such as a transistor, is usually fabricated by a series of diffusion steps. Each diffusion step involves applying a coat of photosensitive polymer, known as photo-resist, over a silicon dioxide layer on the surface of the semiconductor substrate. A photomask is pressed against the surface of the photo-resist and the photo-resist exposed to light. When the photo-resist is photographically developed, selected areas of the photoresist are removed to expose the underlying silicon dioxide. The exposed silicon dioxide is then removed by an etching fluid which does not attack the photo-resist to expose the underlying semiconductor material. The photo-resist is then stripped from the silicon dioxide and impurities diffused into the areas of the semiconductor material exposed by the openings in the silicon dioxide layer. A new silicon dioxide layer is either grown over the exposed portion of the semiconductor material during the diffusion process, or is subsequently deposited, and the procedure repeated for the next diffusion step.

Each successive diffusion is typically made either into only a portion of a previous diffusion, or into a different area of the semiconductor slice so that a different photomask is required for each diffusion step. Each photomask is typically a square of flat glass with a photographically fixed high resolution emulsion on one face which has opaque and transparent areas. Since the face of the photomask carrying the fixed emulsion is pressed directly against 3,539,256 Patented Nov. 10, 1970 the slice, the patterns on the photomask must be actual size, which may involve geometries from as large as tenths of inches to as small as tens of micro inches, although line widths on the order of forty micro inches are generally considered to be the ultimate limit when using silicon dioxide as the masking layer.

Semiconductor material is more easily grown, handled and processed as disk-shaped slices having a nominal diameter of about 1.5 inches and a thickness of about ten milli inches. For this reason, a large number of semiconductor devices are typically fabricated simultaneously on each slice by the same process steps. It is also common practice to fabricate semiconductors, diodes, resistors, and capacitors for a complete circuit on the same semiconductor substrate, and then interconnect the components by leads patterned from a metal film deposited on the surface of a silicon dioxide layer by the same photolithographic process. Openings are provided in the oxide layer where the metal leads must make contact with the individual active components. The fabrication of integrated circuits usually requires a larger number of diffusion steps, and thus a larger number of photomasks for the diffusion steps, and in addition requires an extra photomask to pattern the metal film to form the interconnecting leads. It is also common practice to simultaneously fabricate a large number of integrated circuits on each individual slice of semiconductor material by the same process steps.

It is impractical, if not impossible, to produce a photomask for a large array of either discrete devices or integrated circuits by drawing the entire mask on an enlarged scale and then photographically reducing the entire mask. However, the basic portion of each mask relating to a particular device, group of devices, or an integrated circuit can be originated on a much larger scale, and then optically reduced to a light image of actual size. Then the light image can he stepped over a photographic plate to produce the complete photomask. However, it is vitally important that the light image be precisely located at each successive exposure position with great precision. Otherwise, the successive photomasks will not completely register and the yield will be low.

One method for overcoming this problem involves producing all photomasks of a set simultaneously in a multibarreled step and repeat camera. Then the same positional errors will occur in all masks of the set and the masks will perfectly register. However, this is not practical. Each photomask is good for only a limited number of exposures, for example from twenty to forty. Since a relatively large number of the slices prove defective at an early stage of the process, a much larger number of the photomasks used in the early steps of the fabrication process are required than the number of photomasks used in the latter steps of the mask. Thus, in normal high volume production, the method would result in wasting a large number of photomasks in a short period of time.

Integrated circuits are widely used as the storage ele ments and as the logic gates for digital computers and automated control systems. As a result, large numbers of the individually packaged integrated circuits are often interconnected by printed circuits, or other similar techniques, into a large system. In the last few years, yields have increased to the point where it is practical to fabricate a large' number of integrated circuits on a single slice of semiconductor material, test the circuits in situ on the slice, and then interconnect only the good circuits into an array by one or more levels of thin film leads deposited over the slice. However, from one-fourth to one-third of the circuits on a slice may be faulty, and the faulty circuits occur at random positions on the slice.

This means that a very large number of different combinations of good circuits can result. A customized photomask, or set of photomasks, must therefore be generated to pattern the thin film lead patterns on each individual slice. This would be highly impractical using conventional techniques. The wiring masks can be generated by a computer controlled system. But such a system presupposes that each component or circuit is located at a predetermined position on the slice with considerable accuracy. Otherwise, a short or open circuit may be produced at some point where the lead pattern does not register with the circuits, and the entire arry would then be faulty.

There is, therefore, a pressing need for a system for generating photomasks in which the position of each pattern on the mask is located with an accuracy on the order of a few micro inches. The very best systems heretofore available for positioning the table of a step and repeat camera, or any other movable stage such as those used for automatically positioning machine tools, have positional accuracy on the order of forty micro inches, thus requiring an improvement of about an order of magnitude. Further, prior step and repeat cameras are capable of producing masks only about 1.5 inches square, although semiconductor slices about three inches in diameter are now available. On the order of one thousand individual integrated circuits may be placed on a slice having a nominal diameter of about one inch, and on the order of ten thousand circuits can be placed on a slice having a nominal diameter of three inches without decreasing the circuit size. This large number of exposures would take an extremely long period of time using previous step and repeat cameras.

This invention is concerned with an improved step and repeat camera for automatically generating one photo mask or a complete set of photomasks required for producing an array of integrated circuits, or the like, with a positional accuracy on the order of a few micro inches. This can be achieved by sensing the movement of a film support table in X and Y coordinate directions by means of a laser interferometer and interference fringe counter, and continuously positioning the table in the X and Y coordinate directions by X and Y drive systems controlled in real time by a digital computer. The computer is programmed to compute the current position of the table from the fringe counts, compute the desired position of the table, and compute the force required by the drive system to move the table to the desired position and hold the table in the desired position for as long as required to make the exposure. Control of the table in real time by a computer is possible only if the table is supported by a very low friction support system which also maintains the table precisely within a predetermined plane. In addition, the table must be movable in one coordinate direction without changing its position in the other coordinate direction more than the desired tolerance value.

In accordance with this invention, the table is supported by constant pressure air bearings traveling over a level support surface to provide very low friction and to precisely position the table in the desired plane. In addition, the table is guided along each coordinate axis by a precision guide surface over which a fixed constant pressure air bearing travels to maintain the desired positional accuracy without introducing friction.

These and other novel features believed characteristic of this invention are set forth in the appended claims. The invention itself, however, as well as other objects and advantages thereof, may best be understood by reference to the following detailed description of an illustrative embodiment, when read in conjunction with the accompanying drawings, wherein:

FIG. 1 is a front elevational view of a step and repeat camera constructed in accordance with the present invention;

FIG. 2 is a plan view of the camera of FIG. 1 with the 4 upper stages removed to better illustrate the movable table;

FIG. 3 is a simplified isometric view of the support and drive means for the movable table of the camera of FIG. 1;

FIG. 4 is a front elevational view of a portion of the table of the camera of FIG. 1 partially broken away to reveal details of construction;

FIG. 5 is a plan view, partially broken away, of the portion of the table shown in FIG. 4;

FIG. 6 is an end view, partially broken away, of the portion of the table shown in FIG. 5;

FIG. 7 is a sectional view taken substantially on lines 7-7 of FIG. 5

FIG. 8 is a partial sectional view showing a detail of construction of the portion of the table shown in FIG. 5;

FIG. 9 is a plan view, partially broken away to show details of construction, of a multiple plate carrier for the camera of FIG. 1;

FIG. 10 is a rear end view of the carrier of FIG. 9;

FIG. 11 is a side view of the carrier of FIG. 9, partially broken away to show details of construction;

FIG. 12 is a front end view of the carrier of FIG. 9, partially broken away to show details of construction;

FIG. 13 is an enlarged plan view of one corner of the carrier of FIG. 9;

FIG. 14 is an enlarged sectional view taken substantially on lines 1414 of FIG. 9;

FIG. 15 is an elevational view of the outer face of one of the uprights of the camera of FIG. 1;

FIG. 16 is a sectional view taken substantially on lines 16-16 of FIG. 15;

FIG. 17 is an elevational view of the inner face of the upright shown in FIG. 15;

FIG. 18 is a sectional view taken substantially on lines 18--18 of FIG. 17;

FIG. 19 is a top View of the upright shown in FIG. 15;

FIG. 20 is a sectional view taken substantially on lines 20-40 of FIG. 15;

FIG. 21 is a top view of the upper stage of the camera of FIG. 1;

FIG. 22 is a front elevational view of the upper stage shown in FIG. 21;

FIG. 23 is a bottom view of the upper stage shown in FIG. 21;

FIG. 24 is an enlarged view of the portion broken away in FIG. 22

FIG. 25 is a sectional view taken substantially on lines 2525 of FIG. 21

FIG. 26 is a top view of the lower stage of the camera of FIG. 1;

FIG. 27 is a sectional view taken substantially on line 27--27 of FIG. 26;

FIG. 28 is a schematic block diagram of the control system for the camera of FIG. 1;

FIG. 29 is a more detailed schematic block diagram of a portion of the control circuit illustrated in FIG. 28;

FIG. 30 is a graph illustrating the operation of the portion of the control circuit shown in FIG. 29;

FIG. 31 is a schematic diagram which serves to illustrate the operation of the step and repeat camera of FIG. 1;

FIGS. 32a and 32b, taken together, are a simplified flow diagram of the program of the computer shown in FIG. 28 which is used to control the step and repeat camera illustrated in FIG. 1;

FIGS. 33-41 are flow diagrams illustrating subroutines within the program represented in FIGS. 32a and32b; and

FIG. 42 is a graph which plots the force applied for a given positional error in order to maintain the table at a predetermined position during exposure.

Referring now to the drawings, a step and repeat camera constructed in accordance with the present invention is indicated generally by the reference numeral 10 in FIG. 1. The camera is mounted on a massive concrete block 12 which is suspended from springs (not illustrated) for isolating the block 12 from the vibrations of the earth. The springs are adjustable so that the concrete block can be leveled. If desired, a pneumatic, selfleveling system can be employed. A base casting 14 is mounted on the concrete block 12 by legs 16. A lower granite block 18 is supported on the base casting 14 by three triangularly spaced threaded rods 20 and nuts 24 which rest on casting 14. The upper surface 22 of block 18 is highly planar and is disposed precisely level by adjustment of the nuts 24 on the threaded rods 20 which rest on the base casting 14. A film support table, indicated generally by the reference numeral 26, is movable in X and Y coordinate directions over the planar surface 22 of the lower granite block 18.

A pair of uprights 28 and 30 are connected to the base casting 14 and extend upwardly on either side of table 26. Upper and lower stages 32 and 34 are mounted on the uprights 28 and 30 for adjustable movement in the vertical direction. The upper stage 32 supports a pair of light houses 36 and 38 each of which contains nine light sources. Each light source includes a lamp and a lens system to project light along eighteen separate optical axes. The upper stage has facilities for supporting a master transparency for each optical axis. The lower stage 34 supports a reducing lens for each of the optical axes, and a bellows 40 for each optical axis extends between the upper and lower stages. The table has provision for supporting a photographic plate on each optical axis so that it will be exposed by the image produced by directing light from the source through the respective transparency and reducing lens onto the photographic plate. Each of the transparencies carries the pattern required for a different photomask used for the different steps of the semiconductor fabrication process. When table 26 is indexed to successive exposure positions, all plates carried by the table are simultaneously exposed so that the exposures will have the same positional errors. All of the patterns on the photomask can then be made to register simultaneously. The eighteen separate optical axes permit a set of photomasks for an eighteen step fabrication process to be produced.

An important aspect of the present invention is to be able to position each exposure on each film plate at any desired location within a field of travel several inches square, with a positional tolerance of only a few micro inches. This not only requires positioning of the table 26 within that tolerance, but also dictates that the apparatus 10 be located in a room where the temperature is maintained constant within a fraction of a degree. Otherwise, expansion of the mechanical parts of the camera will move the transparencies or plates by an amount greater than the specified limits. Similarly, vibrations set up in the machine either from the earth or from within the machine may cause elongations and contractions which would result in the inability to meet the tolerances. These problems are compounded by the very large size of the camera 10 required in order to achieve the large field of travel and a high photographic reduction ratio of as much as 20:1.

The table 26 is comprised of a granite block 42 which supports a metal casting 44. The granite block 42 is supported by four conventional constant pressure air bearings 46 which ride on the surface 22 of granite block 18. Each of the air bearings 46 has a planar bottom surface disposed adjacent to the highly planar surface 22 of the lower granite block 18. Gas, typically nitrogen, is pumped under a constant pressure through the center of each air bearing 46 so that the table 26 is continuously supported by a very thin layer of gas, typically on the order of two microns thick. As a result, the table 26 can be moved over the supporting granite block 18 with a minimum of friction. The gas supply and the individual pressure regulator provided for each air bearing are not illustrated.

Movement of the table 26 is precisely controlled by a guide and drive system which includes a first guide means formed by glass bars 48 and 50 which are mounted on the lower granite block 18. The edge faces 48:: and 50a of the bars 48 and 50 are optically flat and are precisely aligned, and the opposite edge faces 48b and 50b are substantially fiat and parallel to the optically flat faces. An inter-mediate stage is formed by granite slabs 52 and 54 which are rigidly interconnected by a third granite slab 56. The slabs 52 and 54 are disposed on opposite sides of the guide rails 48 and 50 and the third slab 56 bridges over bars 48 and 50. Slabs 52 and 54 are supported by pairs of air bearings 53 and 55, respectively, which ride on surface 22 of block 18. A pair of inverted U-shaped yokes 58 and 60 are fixed to the bridge slab 56 and extend downwardly to stand off from the opposite edge faces of guide rails 48 and 50. The yoke 58 carries a fixed air bearing 62, which rides on the optically flat edge face 48a, and a pneumatically biased air bearing 64 which rides on the opposite face 48b and. continually biases the fixed air bearing 62 against 'face 48a with a constant force. Similarly, the yoke 58 has a fixed air bearing 66 which rides on the optically flat edge face 50a, and a pneumatically biased air bearing 68 which rides on the opposite face 50b to continually force air bearing 66 against the reference face with a constant force. Thus the intermediate stage is free to move only in the X coordinate direction and is retained at a predetermined Y coordinate over its entire travel within the design tolerance of a few micro inches.

A second guide means is formed by glass bars 70 and 72 mounted on slabs 52 and 54 and have optically fiat surfaces 70a and 72a which are aligned precisely at right angles to the optically fiat surfaces 48a and 50a. The opposite faces 70b and 72b are substantially flat and substantially parallel to faces 70a and 72a. A second pair of inverted U-shaped yokes 74 and 76 are fixed to opposite edges of the granite block 42, and have fixed air bearings 78 and 80 which ride on the optically fiat surfaces 70a and 72a, and pneumatically biased air bearings 82 and 84 which ride on edge surfaces 70!) and 72b to bias the fixed bearings against the reference surfaces with a constant force.

The granite block 42, and hence the table 26, can be moved in the X direction along guide rails 48 and 50 by means of an X axis drive system comprised of a printed circuit motor 86, which is mounted on upright 28, and drives a wheel 88 which frictionally engages one edge of a drive bar 90. The bar 90 is connected to the intermediate stage by a rod 92. A pair of idler rollers 94 are spring biased against the opposite edge of drive bar 90 to maintain a substantially constant force between the drive wheel 88 and the drive bar 90. The opposite end of the drive shaft of printed circuit motor 86 is provided with a pneumatically operated disk brake which is represented schematically at 96.

The granite block 42, and hence table 26, can be moved in the Y coordinate direction by a second printed circuit motor 98 which drives wheel 100. Wheel 100 frictionally engages one edge of a bar 102 which is connected to yoke 76, and therefore to block 42, by a rod 104. The Y axis drive motor 98 and idler rollers 106 are mounted on slab 54 by a suitable means represented by bracket 108. A pneumatically operated brake 110 is also provided on the shaft of the printed circuit motor 98. Thus printed circuit motor 86 moves the table 26 in the X coordinate direction, and printed circuit motor 98 moves the table in the Y coordinate direction. As will hereafter be pointed out in greater detail, the brakes 96 and 110 are used only when the system is not in operation and are not used to position the table during exposure.

The casting 44 of the table 26 is shown in detail in FIGS. 4-8. The casting 44 is adapted to receive a pair of multiple plate carriers, each indicated generally by the reference numeral 120, in precisely predetermined positions relative to the optical axes. As will hereafter be evident, at least four of the film plate carriers 120 are required for full operation of the camera system. One of the film carriers 120 is illustrated in detail in FIGS. 9-14. Each film carrier is comprised of a base plate 122. A peripheral side wall 124 is integral with the base plate 122 and extends around the entire periphery of the base plate. A lid 126 is connected to the peripheral side wall 124 by hinges 128 and 130.

The carrier 120 has nine identical compartments formed by interior walls 132, which are also integral with the front plate 122, and the peripheral side wall 124. Aligned square openings 122a and 126a are provided in the base plate 122 and lid plate 126 at each compartment to permit light to be projected through a film plate disposed in the compartment. Each compartment is adapted to receive a standardized square glass photographic plate 134 and to hold the plate in a precisely oriented position relativeto the carrier. Orientation longitudinally of the optical axes, which may be considered the Z axis, and also pitch orientation about the X and Y axes, is provided by three studs 136 which project into each compartment from the base plate 122. The film plate 134 is oriented along the X and Y directions, and also in rotation about the Z axis, by a pair of banking lugs 138 and 140 which are pivotally mounted on pins 142 and 144, respectively, and a third lug 146 which is pivotally mounted on a pin 148. The edges of lugs 138, 140 and 146 are straight along the dimension extending longitudinally of the edge of the film plate so as to engage the edge of the plate 134 along a substantial distance, but are rounded in the direction normal to the film plate so as to engage only the center of the edge of the film plate 134. This curvature can best be seen in FIG. 12.

Two adjacent edges of the film plate 134 are biased against the banking lugs 138, 140, and 146 by an assembly comprised of springs 150 and 152 and an elbow shaped member 154 which engages the corner of the film plate opposite the edges which abut the banking lugs. The elbow-shaped member 154 is retained in position when the film plate 134 is removed by a pin 156 which is received in an oversized hole (not illustrated) in the elbowshaped member 154. The pin 156 has a head larger than the oversized hole to retain the member 154 in place on the pin.

Each film plate 134 is urged downwardly against the three positioning studs 136 by leaf springs 160 which are carried by the lid plate 126 and engage the glass plate 134 directly over each of the studs 136. The lid plate 126 is held against the cumulative force of the leaf springs 160 by a pair of fasteners 161. Each fastener is comprised of a strap 162 which is fixed to the lid plate 126. An aperture 164 in each strap receives the rouned end of a stud 166 which is slidably disposed in the side wall 124 and is biased outwardly by a leaf spring 168.

Each of the film plate carriers 120 has a pair of flat banking surfaces 170 and 172 formed on the outer surface of one peripheral side wall 124, and a third banking surface 174 formed on the adjacent side wall. The banking surfaces 170, 172 and 174 on each carrier 120 are in precisely predetermined relationship to the banking lugs in each compartment of the carrier. The base plate 122 has a number of precision ground reference surfaces 176, shown in dotted outline in FIG. 9, which lie in a common plane. The ends of all of the blanking studs 136 in all of the compartments also lie in a common plane disposed parallel to the plane of the reference surfaces 176. A handle 180 is attached to the peripheral wall 124 to facilitate handling the carrier 120.

The film plate carriers 120 are used to carry both the unexposed film plates for table 26, and also the master transparencies, for the upper stage 32 through which light is projected. The carriers are easily loaded merely by placing the carrier on a table with the reference surface 176 down, releasing the latch assemblies 161, and raising lid plate 126. The film plates 134 may then be placed in the respective compartments merely by manually moving the elbow-shaped member 154 against the force of springs and 152, placing the plates on the studs 136 with the photosensitive emulsion or the phototransparency face down, and releasing the elbow-shaped member 154. The springs 150 will then bias the plate 134 securely against the banking lugs 138, 140 and 146 to precisely position the plate relative to banking surfaces 170, 172 and 174. After all of the plates 134 are loaded in the respective compartments in this manner, the lid plate 126 is closed and the fasteners 161 latched so that the springs securely hold the plates 134 in place against reference studs 136 which precisely orient the plates parallel to the plane of reference surfaces 176. The plate carrier 120 can then be easily taken to the camera 10 and inserted as will pres ently be described.

Referring once again to FIGS. 4-8, the casting 44 of table 26 is adapted to receive a pair of the plate carriers 120, although only one carrier is illustrated in the drawings. The casting 44 has a W-shaped top plate 200 forming two openings 202a'and 20% each of which is slightly larger than the array of nine openings 122a in the base plate of a carrier 120. The carriers 120 can be inserted into openings in the front of the casting 44 and be slid into position beneath openings 202a and 20% on rails 208 and 210 extending along opposite sides of the openings. A pair of banking lugs 212 and 214 are pivotally mounted on pins 216 and 218 at the right hand side of each of the openings 202a and 20212 and are spaced to engage the machined reference surfaces and 172, respectively, on a carrier 120. A third banking lug 220 is pivotally mounted on pin 222 at the rear of each opening 202a and 202b and is spaced to engage the reference surface 174 on the carrier 120. The carrier 120 is biased against the banking lugs 212, 214 and 22 0 by a lever 224 which is pivoted on a pin 226 and is biased against the corner of the carrier 120 by a coil spring 228 as shown in the detail of FIG. 8.

Four adjustable alignment studs 206 are disposed around each of the openings 202a and 202b, although only three are shown around opening 202 in FIG. 5, and extend downwardly through the top plate 200 and terminate in a common plane parallel to the surface 22 of the granite block 18. A hand operated mechanism is provided to raise each of the carriers 120 from the rails 208 and 210 to hold it tightly against the respective sets of downwardly projecting alignment studs 206. This mechanism is best illustrated in the right-hand section of FIGS. 4 and 5 and in FIGS. 6 and 7. A pair of shafts 230 and 232 are journaled in side walls 234 and 235 and an intermediate rib 236 of the casting 44. Throws 244 and 246 are fixed to shafts 230 and 232, respectively, by pins, and a box beam 238 is pivotally connected to the throws 244 and 246 by pins 240 and 242. A pair of spring supported stools 248 and 250 extend upwardly from the beam 238. A second beam 252 is identical to the beam 238 and extends between throw pins (not illustrated) on throw plates 254 and 256 which are mounted on shafts 230 and 232. A second pair of spring biased stools 258 and 260' extend upwardly from beam 252. A connecting rod 264 which is pivotally connected to throw plates 244 and 246 by pins 266 and 268, and an identical connecting rod 27-0 is pivotally connected to throw plates 254 and 256 so that shafts 230 and 232 can be simultaneously rotated by manipulating a hand lever 262, which is keyed on shaft 230. The lift mechanism associated with opening 202a is identical, except that the hand lever 262 is mounted on shaft 232. When hand lever 262 is moved to the position illustrated in the right-hand half of the figures, the stools 248, 250, 258 and 260 are lowered so that the carrier 120 rests on slide rails 208 and 210. When the lever 262 is 

